Firewalls not affected by this vunerability now include Tiny Software's Tiny Personal Firewall version 3, which was released last week, includes a new application "sandbox" feature, is not vulnerable to programs like this. Additionally, the popular ZoneAlarm personal firewall is also not susceptible to the attack, according to Iorio.

Last November, security researchers published several techniques for evading some firewalls' guards against unauthorized leaks. Tools named TooLeaky and FireHole demonstrated how attack programs could piggy-back on applications with approved access to the Internet.

Iorio said Backstealth is unique because it does not commandeer a trusted program, but instead uses a Windows function called VirtualAlloc to inject itself into the firewall's memory space.

"Hackers are always going to come out with new ways to get around firewalls. But they all rely on executing code on your system. And that means they can be detected by anti-virus software," if the programs perform malicious activity, said Symantec product manager Tom Powledge.